Variable attenuator



June 6, 1950 E. WEBER ET A1.

VARIABLE ATTENUATOR 'Filed March 50, 1945 Patented June 6,` 1950 UNITED STATES PAT ENT OFFICE,

VARIABLE ATTENUATOR Emst 'Weben Mount Vernon, and John W. E.

Griemsmann,.ellaire, N. Y.,.assignors to :Polytechnic- Institute of Brooklyn, Brooklyn, N. Y., a corporation of New York Application March 30, 1945, Serial' No; 585,656

This invention relatesto variable attenuators for Ihi'gh' frequency currents and especially vfor currents having frequencies in the'vultra-high yfrequency'range andV higher.

For Amany applications in laboratory measurelments `as well as in testv equipment' in the eld, fitni's necessary toV havevariable attenuators availu able in order to either measure power output of imbesasv oscillators or to regulate the powerflow 'into special equipment.

An object of the invention is to devise-a variable attenuator' for 'high 'frequency currents and having abroad-band1 characteristic.

Af'urth'er object oi the invention is todevise a variable attenuator which -isstable in its op- Veration and accurate in its performance.

Broadly the invention involves a transmission vconductor having an lattenuator section of relatively high resistance bycompar-ison with the adjacent sections and providing the necessary maximum attenuation, The high resistance section'isformedrwith'arwallthickness preferably-less than the depth of penetration (skinfdepth). A conductive shunting memberlislarrangedlin close- 'l'y'-'spaced rrelation vwith the attenuator sec conductor' andthe movable shuntingmexnber would be a flat plateelike element. Y

Y The variable attenuatorsgoi the present invention involve the use of metal-nlm attenuator units' of the ktype-*disclosed Vinizo-pending applications' Serial NO. 540,347", filed June 14, v1942i, and Serial No. 581,195, ledMarchfG, 1945.l

Two d'iierent-eml'oc'dimentsof the invention areillustrated in the accompanying drawing in which: Y

Figure 1 lis a longitudinal sectional View oi one "cons'tructionof a variableV attenuator;

Figure 1a is an enlarged sectional view-fof part of the attenuator unit showing theoper'ation 'of the matching cone on the -shuntingplunger Figures lb-and :1c are Vfr-agmfeii-tary' views showing modied forms'of plungers'g' 'Figure v2 is aiongitudinai sectional view snowing smotheri'orrnoffv attenuator construction yann- Figure 3 is a sectional along theA line 3-3.

The attenuator illustrated in Figure lis con'- structed as a unitary assembly -which 'may `be inserted in'a coaxial line; Inthis arrangement the attenuator unit is mounted within a cylin- `d'rical casing formed of two'linear sections l and la of a metallic tube detachably joinedV together by a suitable coupling Ib. 'Theattenuatorunit 2 is mounted at the center -of the casing and forms a linear section of the-center conductor of the coaxial cable. The attenuator unit is constructed in accordance with copending application Serial No. 540,347, and is formed of 'an-insulating tube having bullet type connectors 2a and 2b at the ends thereof which engage sockets formed in the ends of centerv conductor' sections 3a andBb mounted in the 'two sections of the attenuator casing. A'thin metallic lm is depositedv on the outer surface of' the linsulating tube to provide the required attenuationin the currenttransmitted through the attenuator assembly. The attenuator film is Very thin, vpreferably only a fraction of the depth of penetration (skin depth), although greater thicknessesmay view of Figure 2 taken be used vup vto twice the depth of penetration.

The film on the end sections ofthe tube indicated `rat 2ay and 2b' is Amade thickerthanth-e lm on Vthe central section of'the attenuator unit, thus providing compensating sections on, the ends of the attenuator unit to prevent wave reflections from the attenuator, as more fully described in copending application Serial No. 581,195, filed March 6,1945. The'supplyrcable (notshown) may be connected either tothe left end oftubular section la orto ythe end of a stub extension la'. As shown in Figure 1, the supply cable would be connected Vtoithe stubext'ensionfia, and the left -end of section vla is closed by a plug I'c which `is sopositioned that its inner face is locatedat eiTectivelyone-quarter of a Wave length from the axis ofstub lextension la. In case the supply cable is connected. to the left endof section la, the-plug Ic would be removed and placed' in the end of extension I aand located so 'that its inner face will be positioned at eiTectively one-quarter of a wave length from the axis of the center conductor 3a. A center conductor section 30, is `mounted Within. stub vlol and connected with "the y'center conductor section 3a.

The outgoing cable or the apparatus to be tested is connected to a stub extension'fld on 'tubular section I, andthe rightend of section -l vis rclosed by a Yplug le, the inner face of which 'iswlocatedat effectively oneequarter-:of a wave 3 length from the axis of stub I d. A center conductor section 3b is mounted within stub Id and is connected with center conductor section 3b. That part of casing section I extending to the right of stub Id, and the corresponding part of inner conductor 3b constitutes eiectively a quarter-wave stub support for the right end of the attenuator unit 2 and center conductor 3b.

It will be understood that the connections of the two cables may be reversed, that is, the incoming cable may be connected to extension Id and the outgoing cable to extension Ia'.

In order to provide for variation of the attenuation produced by the attenuator unit 2, the center conductor section 3b, and the bullet connector 2b of the attenuator unit are provided with a center bore in which is mounted a metallic plunger 4 formed as a metallic rod or tube having an outside diameter which will freely slide within the insulating tube carrying the attenuator nlm. The right end of plunger ll extends beyond the end of the attenuator casing and is vprovided with a detachable head 5. A rack 6 is arranged on the outside of the casing I parallel with plunger 4 and is connected at one end to the plunger head 5. This rack passes through a slot formed in block 1 carried by tube I and is engaged by a driving gear 8 mounted on a shaft 8a journaled in the block 1. By rotating the shaft 8a, the plunger may be moved in and out of the attenuator unit to various positions through the action of gear 8 and rack 6. It will be understood, however, that rack 6 and gear 8 are not essential and the plunger may be moved manually by grasping the end of the plunger or by grasping the head 5.

By providing an index pointer 6a, on rack 5, the attenuator may be calibrated in decibels marked on a scale arranged parallel with the path of travel of the pointer Sa, for example, the scale may be inscribed on the outer surface of tube I, or it may be inscribed on a separate element. Various other arrangements for driving the plunger are possible as will be understood by those skilled in the art.

As the plunger 4 moves into the attenuator unit, the end of the conductive plunger to the left of the bullet connector 2b provides a parallel path for the current flowing through the attenu ator unit, and the plunger, in effect, serves to shunt a variable part of the attenuator film. Accordingly, the amount of attenuation produced by the attenuator unit may be varied by varying the position of the plunger 4 within the attenuator unit. Lowest attenuation is obtained when the plunger is fully inserted into the unit, and greatest attenuation is obtained when the plunger is fully withdrawn from the unit.

For the purpose of preventing reflection from the end of the plunger 4, a matching element 4a in the form oi a conductive cone is attached to the end of the plunger 4. This cone may be formed as a separate element attached to the plunger or it may be an integral part of the plunger. The matching cone has a length almost equal to, but somewhat less than, one-quarter of the wave length of the mid-band frequency. The diameter of the cone at the small end is as small as possible but must have enough mechanical strength for self-support. The cone 4a, acts as an antenna-like attachment to the plunger for guiding the energy out from the plunger to the lm, or from the nlm to the plunger, and causing a gradual transition of energy flow from the attenuator film into the conductive plunger, or vice Versa. Figure la is an enlarged sectional View of a part of the attenuator unit showing how the cone 4a acts as a transition element for gradually transferring the spherical waves from the plunger 4 to the attenuator lm on the outer surface of tube 2.

In use of the movable plunger in actual practice, the plunger need not be withdrawn so that the end of section 4 of the plunger passes beyond the inner boundary of the matching section 2b'. In case accurate matching is not required, the matching section 2b' could -be omitted, and in this case the plunger may be withdrawn until the end of section 4 reaches the inner end of bullet connector 2b.

Other arrangements for providing a matching section on the end of plunger 4 are possible. For example, the plunger might be formed of an insulating tube which slides freely within the attenuator unit and the main conductive section 4 of the plunger would be formed of a metallic coating or lm carried by the tube, while the matching section 4a would be formed of a linear section of coating or lm having a higher resistance than the main section 4. Such an arrangement is shown in Figure 1b. Also, the lm on the matching section could be constructedso that it changes in resistance gradually along its length, having a low value at the point of the junction with the section 4 and increasing in value to the end of section 4a. Finally, the matching section on the end of the plunger could be formed as a series of wedge-shaped strips of metallic lm having their bases in Contact with the section 4` and extending to the end of section 4a as shown in Figure 1c.

The broad-band characteristic of the attenuator is due mainly to the matching element or transition element carried by the shunting plunger and to the matching sections at the ends of the attenuator lm, and especially the matching section 2a.

Figure 2 is a fragmentary showing of a modified arrangement of attentuator unit for incorporation in a straight section of a coaxial cable. Elements having the same functions as corresponding elements in Figure 1 are represented by the same reference numerals. In this arrangement I indicates the outer tubular casing of the attentuator assembly which is connected to the outer conductor of the supply cable not shown, and 3 indicates the inner conductor which in this case is also hollow. 3a and 3a indicate a T-shaped mounting for supporting the inner conductor in a stub extension I' of casing I. The length of the stub is fixed at effectively onequarter wavelength to prevent wave reflection from the mounting. The attenuator unit 2 is supported in line with the head 3c of the mounting, and this head is formed with a central bore in which is mounted the plunger 4 which may slide to the right into the attenuator unit 2 or to the left into center conductor 3. A rack is formed on one side of the plunger 4, or a separate rack may be secured to the plunger, and this rack is engaged by a pinion 9 carried by a shaft Il) mounted in a central bore formed in the support 3a and extending out of the stub I'. Shaft ID has a driving wheel or gear Illa mounted on the end thereof which may be rotated by hand or through any suitable driving arrangement.

The arrangement shown in Figure 2 may involve any of the structures described above for the matching section 4a. From the foregoing description of operation of Figure 1 it will be apparent that the plunger 4 may be advanced into film on the outer surface thereof, said film having a thickness of the order of the'depth of penetration of said currents and presenting substantial resistance to the flow of said currents, and a conductive plunger mounted for movement within said tube but out of contact with said film.

2. A loss-producing variable attenuator for high frequency currents comprising a thin metallic film in tubular form and having a thickness of the order of the depth of penetration of said currents and presenting substantial resistance to the flow of said currents, and a conductive plunger arranged for movement into said tubular film but out of contactl therewith.

3. A variable attenuator for high frequency currents comprising a tubular conductor having a high resistance section formed of a thin metallic film joined to a low resistance section, a conductive plunger mounted coaxially with said tubular conductor and arranged so that one end thereof may slide into said high resistance section, and means embodied in said plunger at the end thereof for preventing wave reflection from the joint between said high resistance section and said low resistance section.

4. A variable attenuator structure for high frequency currents comprising a conductor having a low resistance tubular section and a high resistance tubular section having a thickness of the order of the depth of penetration of said currents, and a conductive plunger mounted within said low resistance tubular section and being arranged to be moved into said high resistance tubular section but out of contact therewith'.

5. A variable attenuator according to claim 4 wherein said high resistance section of the conductor has a wall thickness less than the depth of penetration.

6. A loss-producing variable attenuator for high frequency currents comprising a tubular high resistance conductor section having a wall thickness of the order of the depth of penetration of said currents, and a conductive plunger arranged to slide within said tubular conductor section but out of contact therewith to provide a transmission path in parallel with a variable portion of said tubular conductor section.

7. A variableV attenuator for a coaxial cable in which the center conductor of the cable is supported by an effective quarter-wave stub mountin-g, the combination of a tubular section of the inner conductor of said cable located adjacent said mounting and having a wall thickness of the 6 order of the depth of penetration, a conductive plunger arranged to slide within said tubular section to provide a transmission path in parallel with a variable portion thereof, and means located within the center support of said stub mounting and extending out of the endof said mounting for adjusting the position of said plunger.

8. A variableY attenuator according to claim 7 wherein the cable sections on opposite sides of said stub mounting are in axial alignment and said stub mounting is arranged at right angles t0 the axis of said cable, and wherein said conductive plunger is shifted axially of said cable by means of a rack and pinion, the pinion having a rotary shaft journaled in the center support of said stub mounting and extending out of the end of said mounting.

9. A variable attenuator according to claim 7 wherein they cable sections located on opposite sides of said stub mounting are arranged at right angles to each other, and said stub mounting is arranged in axial alignment with one of said sections, and the conductive plunger of the attenuator is provided with a rod extension passing through the tubular center support of said stub mounting and extends beyond the end of said mounting.

10. A variable attenuator structure for high frequency currents comprising a conductor having a low resistance tubular section and a high resistance tubular section joined to said low resistance section, and a conductive plunger mounted within said low resistance tubular sectionand being movable into said high resistance tubular section Without contact therewith, and means embodied in said plunger at the end which enters said high resistance section for preventing wave reflection from the joint between said two conductor sections.

11. A variable attenuator according to claim l0 wherein said reflection preventing means comprises an end section of said plunger formed into generally conical shape with the base thereof located at the end of the plunger.

12. A variable attenuator according to claim l0 wherein said reflection preventing means comprises an end section of said plunger having a diierent characteristic impedance from the main body of said plunger.

ERNST WEBER. JOHN W. E. GRIEMSMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

